Spray polyurea systems can be prepared by mixing or combining together a diphenylmethane diisocyanate (xe2x80x9cMDIxe2x80x9d)-containing prepolymer and at least one amine resin. Spray polyurea systems prepared from MDI-containing prepolymers having a 2,4xe2x80x2 MDI-isomer content of about greater than or equal to 25% by weight, and, more preferably, greater than or equal to 30% by weight (hereinafter xe2x80x9chigh 2,4xe2x80x2-isomer MDIxe2x80x9d) have several characteristics that coating applicators find attractive. The high 2,4xe2x80x2-isomer MDI content prepolymers have increased storage stability, lower viscosities, and slower reactivity. The slower reactivity allows the spray applicators to use less expensive equipment to spray the polyurea coatings. Spray polyurea coatings prepared with high 2,4xe2x80x2-isomer MDI content prepolymers possess a smoother surface, increased elongation and lower modulus. High 2,4xe2x80x2-isomer MDI content prepolymers also have some negative effects on spray polyurea systems. The coatings obtained from such systems have lower tensile strength and lower tear resistance than coatings prepared from relatively low 2,4xe2x80x2-isomer MDI content prepolymers. The high 2,4xe2x80x2-isomer MDI coatings also develop physical properties at a slower rate.
Coating formulators currently blend a relatively low NCO prepolymer with a high 2,4xe2x80x2-isomer MDI and optionally propylene carbonate to obtain a xe2x80x9cquasi-prepolymer.xe2x80x9d This blend solves several processing problems. The low NCO prepolymer generally is prepared from a difunctional MDI and a difunctional polyol. The high 2,4xe2x80x2-isomer MDI that is blended with the low NCO prepolymer may often have a 2,4xe2x80x2-isomer content of about 30 to 56% by weight, preferably about 50 to 56% by weight, and more preferably about 56% by weight. This method of blending prepolymers is currently the preferred method in the industry. The quasi-prepolymer is then combined or mixed with at least one amine resin to form the spray polyurea system. The initial low NCO prepolymers are generally made from a base MDI isocyanate having a 2,4xe2x80x2-MDI content of at least about 25% by weight of the base isocyanate.
Uretonimine modified MDI has also been used to solve processing problems. A prepolymer made from uretonimine modified MDI with propylene carbonate back-added has increased gel and tack free times. With the exception of elongation, this prepolymer, when combined with at least one amine resin, produces excellent polyurea coatings.
The present invention relates to spray polyurea elastomers and systems and methods for making the same. The spray polyurea systems are prepared by a method comprising the steps:
A) providing a first isocyanate composition of at least one diphenylmethane diisocyanate isomer, the composition containing greater than 75% by weight 4,4xe2x80x2-MDI;
B) providing a first isocyanate reactive organic material of one or more compounds containing a plurality of isocyanate reactive groups;
C) providing a second isocyanate composition of a mixture of two or more diphenylmethane diisocyanate isomers, wherein the mixture contains at least 25% by weight of 2,4xe2x80x2-MDI;
D) forming an isocyanate group terminated intermediate prepolymer by reacting the first isocyanate reactive organic material with a stoichiometric excess of the first isocyanate composition, wherein the prepolymer contains from 1 to 13% by weight of free isocyanate groups;
E) mixing the intermediate prepolymer with the second isocyanate composition to produce a blended quasiprepolymer composition having a final free isocyanate group content of from 12 to 28% by weight;
F) providing a second isocyanate reactive organic material comprising at least one compound which contains two or more isocyanate reactive amine groups; and
G) combining the quasiprepolymer composition and the second isocyanate reactive material under conditions suitable for the formation of an elastomer comprising a plurality of urea linkages.